1. Field of the Invention
This invention relates to an electrical circuit for driving an inductive load, and more particularly to an improved electrical circuit for driving an inductive load which is protected against destruction from high counter electromotive force induced between both ends of an inductive load.
2. Description of the Prior Art
An inductive load is used, for example, as an ignition coil for an automotive car, a rotor of an alternator and so on.
Such an inductive load is generally driven by an output transistor connected to the inductive load, as shown in the U.S. Pat. No. 3,640,260. A current runs through the inductive load when the output transistor is conductive, and a current is cut off when the transistor is rendered nonconductive. When the output transistor is rendered nonconductive, counter electromotive force is induced between both ends of the inductive load. This counter electromotive force has been found to destroy the output transistor.
In a prior electrical circuit for driving an inductive load, a suppressor diode is connected between both ends of the inductive load in order to protect the output transistor against the destruction by the counter electromotive force.
The prior electrical circuit, however, exhibits some unfavorable characteristics, as nextly noted.
Firstly, more reverse current runs through the suppressor diode due to the counter electromotive force than the forward current which runs through a collector-emitter path of the output transistor when the output transistor is conductive; therefore the suppressor diode is undesirably too big. Accordingly it is difficult to form the suppressor diode in an integrated circuit device, and the overall cost of the electrical circuit for driving an inductive load becomes prohibitive.
Secondly, when the output transistor is formed in an integrated circuit device, a parasitic diode, which is forward connected to the suppressor diode, is usually formed between a collector of the output transistor and the substrate of the integrated circuit device. Therefore large current runs through a parasitic diode-suppressor diode path and destroys the integrated circuit device itself, when a reverse voltage due to, for example, a surge on an electric power line connected to the path or an erroneous connection of an electric power source is applied between both ends of the path.